Titanium dioxide (TiO.sub.2) is a white powder found in two crystalline forms, anatase and rutile. It has a wide variety of uses including opacifying agents; paint pigments; as an additive to paper, rubber, plastics, cosmetics, floor coverings, glassware and ceramics, printing inks, welding rods and the like. Specifically, TiO.sub.2 may be used as a component in plastics, paper and paperboard, and coatings that may come in contact with food. The amount of TiO.sub.2 that is considered safe is determined by the FDA as an amount up to a maximum of about 20% of the material to which it is added.
Generally, TiO.sub.2 is derived from ilmenite or rutile. For example, one method is to treat ilmenite with sulfuric acid to produce TiO.sub.2 in anatase form. Another method is to chlorinate rutile followed by the conversion of titanium tetrachloride to the rutile form by oxidation. These methods for making TiO.sub.2 require the use of harsh chemicals and conditions, notably by the use of acids and bases.
Various other methods for preparing titanium dioxide are known in the art. For example, in U.S. Pat. No. 4,241,042, there is disclosed a method for preparing TiO.sub.2 which comprises preparing a liquid aerosol of a hydrolyzable Ti(IV) compound, and then reacting the resulting aerosol with water vapor in order to obtain hydrated dioxide which, after drying at a temperature of 120.degree. to 130.degree. C., is converted to crystalline phase anatase. If the reaction is conducted at higher temperatures, of the order of 1100.degree. C., it is converted to rutile.
An alternate method in the '042 patent is to avoid the mixing step of the invention of the '042 patent. This alternate method is described in U.S. Pat. No. 4,574,078.
TiO.sub.2 may also be prepared by using homogeneous precipitation, as described in "Preparation and Mechanism of Formation of Titanium Dioxide Hydrosols of Narrow Size Distribution", E. Matijevic, M. Budnik and L. Meites in "Journal of Colloid and Interface Science" Vol. 61, (2), 1977. This process involves hydrolysis of TiCl.sub.4 solution at about 100.degree. C. in the presence of sulfate ions. The TiCl.sub.4 is acidic due to the hydrochloric acid.
TiO.sub.2 is prepared in the form of spherical particles. See U.S. Pat. No. 4,803,064. The '064 patent relates to treating Ti(IV) with strong acid in the presence of cationic polyelectrolytes having a high molecular weight.
In the art, the recovery of TiO.sub.2 from solution also requires harsh reactive conditions, as adjustment of the solution using strong acidic and alkaline solutions is typically needed. For example, U.S. Pat. No. 5,282,977 teaches a method for separation of heavy metals from waste water by lowering the initial phase to a pH of about 3 using dolomite brick powder. The '977 patent teaches the use of calcium hydroxide and/or sodium hydroxide to raise the pH.
Various other processes for recovering metal oxides also require a pH adjustment using harsh reaction conditions. For example, DE 27 29 756 A uses sulfuric acid to precipitate the metal salt prior to recovering it. U.S. Pat. No. 3,016,286 teaches a two-stage recovery process in which ammonia is used to precipitate the metal ions. These patents are incorporated herein by reference.
Conventionally, a preferred method for recovering TiO.sub.2 is to adjust the slurry solution with a strong acid or base. Generally, hydrochloric acid and sodium hydroxide are used. One reason why the pH must be adjusted is to accommodate the use of aluminum sulfate. This step is highly undesirable due to the harsh reactive conditions, but necessary in the prior art procedures.
Subsequently, the aqueous slurry that has been treated with acid and/or base is then treated with aluminum sulfate and a polymer, preferably an anionic polymer as a flocculant to clarify the solution prior to processing in the waste treatment area. The entire process may be repeated to achieve maximum purification and to maximize the recovery of the heavy metal.
Separation of suspended solids from aqueous suspension has been known in the art. U.S. Pat. No. 5,112,500 teaches the use of incompatible, counterionic organic flocculants. Treatment of low concentration turbid water by forcible sequential additions of anionic and cationic coagulants has been taught in U.S. Pat. No. 5,055,200. However, there has been no teaching or suggestion in the art for recovery of metal oxides without use of the harsh reactive conditions resulting from the addition of strong acids and/or bases.
There is thus a need in the industry for a process of recovering metal oxides, especially titanium oxides, from slurry that does not require the use of harsh chemical treatment, for example, by a process that does not require the use of strong acids and strong bases.
The present invention describes a process for the recovery of metal oxides that uses highly charged polymeric emulsions. Unexpectedly, the novel process of this invention does not require pH adjustments which create harsh conditions. In addition to the advantage of not having to control the pH, which is a safety benefit and a cost saving benefit provided by this invention, the present process also performs better than the conventional recovery processes.